In the transmission of data between a first data terminal equipment (DTE) and a second DTE over a telecommunication line, every DTE is associated with a DCE or modem which modulates and demodulates a carrier signal that is conveyed on the telecommunication line. In order to compensate the frequency distortion introduced by the impairments of the telecommunication line, every modem particularly includes an equalizer. Two major types of equalization systems are well-known in the art.
A first known type of equalization system includes a filter which is designed in order to compensate the distortion provided by a line considered as being a typical one. That typical line is chosen in such a way that its characteristics are approximately similar to the characteristics of a range of actual lines to which the DCE is likely to be connected. However, the drawback involved by that first type of equalization system consists in that the compensation of the frequency distortion is, at most, only an approximation.
A second type of equalization system known as adaptive equalizers is based on the use of an adaptive filter, the transfer function of which is adjusted at the beginning of the transmission of data in such a way that it is accurately inverse of that of the line. Adaptive equalizers therefore provide an accurate compensation of the distortion introduced by any actual line. Such equalizers generally involve the use of a training sequence transmitted over the telecommunication line and which is known from the receiving modem. The analysis of the received training sequence by the receiving modem allows the latter to measure the characteristics of the telecommunication line and therefore adjust the transfer function of the equalizer included in the receiving DCE for a accurate compensation of the distortion. Examples of such equalizers are shown in U.S. Pat. No. 4,047,013 and U.S. Pat. No. 4,089,061 patents assigned to the assignee of the present application and respectively entitled "Method and apparatus for fast determination of initial transversal equalizer coefficient values" and "Method and apparatus for determining the initial values of the coefficients of a complex transversal equalizer". The drawbacks of the adaptive equalization systems based on training sequences firstly consists in the fact that they involve complex technology and a large number of components. Secondly, in some particular applications, for instance in tailing mode or for use in digital networks, a training sequence in not ever possible. Indeed, in tailing mode, multiples tributary stations are connected to a controlling station via a tailed telecommunication network. Whenever an additional tributary station is connected to the network, the latter station is not allowed to request the transmission of a training sequence and thereby disturbing the communication between the already existing stations. The equalization system of the additional DCE consequently needs to be adapted to the characteristics of the network without any training sequence. Similarly, when a DCE is connected to a digital network, for instance Digital Data Services in the United States of America, the latter DCE has to adjust the parameters of its equalizer without any possibility of requesting a training sequence.
Therefore a need has appeared in the telecommunication field for a simple adaptive equalizer which would not require any training sequence and which nevertheless would provide an effective compensation of the distortion introduced by a telecommunication line.